Recognition anionic substrates

Currently, the only other monoprotonated sapphyrin-monoanion complex to be solved by X-ray diffraction analysis is that of 3-HN,. As expected, in this complex the azide counteranion is bound above the sapphyrin plane by a combination of anisotropic electrostatic interactions and oriented hydrogen bonds (Figure 4). As such, this structure supports the conclusion, reached in the case of 3-HCl, that a single positive charge on the sapphyrin is enough to effect anion recognition of anionic substrates, at least in the solid state. 

In conclusion, the bis-barium complex of 17 catalyzes the ethanolysis of anilide and ester substrates endowed with a distal carboxylate anchoring group (Table 5.7). The catalyst shows recognition ofthe substrate, induces fairly high reaction rates with catalytic turnover, and is subjected to competitive inhibition by carboxylate anions, as... 

Anion Coordination Chemistry and the Recognition of Anionic Substrates... 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

Polyaza macrocycles in their protonated form, either full or partial, are able to interact with anionic species, by establishing noncovalent interactions (both electrostatic and hydrogen bonding). Complexation of anions by synthetic polyammonium molecules has been explored over the past two decades, and several examples of selective recognition of different anionic substrates have been reported [14 22], Some polyammonium macrocycles such as [24]aneN6H6 +, 16, and... 

Lehn, J.-M. Meric, R. Vigneron, J.-P. Bkouche-Waks-man, L Pascard. C. Molecular recognition of anionic substrates. Binding of carboxylates by a macrobicyclic coreceptor and cryrtal stmcture of its supramolecular cryptate with the terephthalate ion. J. Chem. Soc., Chem. Commun. 1991. 2762-2771. 

In order to further develop the coordination chemistry of anions and to extend recognition of anionic substrates beyond the spherical halides, an ellipsoidal macro-bicyclic cryptand Bis-Tren (14) was designed, whose hexaprotonated form was expected to bind various anions [9, 10]. Indeed, potentiometric and spectroscopic measurements showed that (14)-6H complexes a number of monovalent and polyvalent anions. The strong and selective binding observed for the linear triatomic anion NJ may be attributed to its complementarity to the molecular cavity of (14)-6H . As confirmed by crystal structure determination, NJ forms the cryptate [N c (14)-6H ] (15), in which the substrate is bound inside the cavity by two pyramidal arrays of three hydrogen bonds, which hold the two terminal... 

A number of years ago, one of the first indications that the synthetic polyammonium macrocycles would be able to go beyond simple recognition of anionic substrates was published by Lehn, Mertes, and coworkers in 1983. The excitement surrounding the discovery of this chemistry was that these simple cyclic systems could mimic the naturally occurring ATPases by catalyzing the hydrolysis of ATP at physiological pHs in aqueous solution. The utility of these simple macrocycles can be attributed to several aspects (i) they operate in water, (ii) due to their cyclic nature the hosts are polyprotonated under physiological pH conditions, (iii) one of the reaction pathways involves a covalent intermediate as a result of the presence of an amine nucleophile, and (iv) the rates of reaction are enhanced upon the addition of certain metal ions as seen for the Mg +-dependence of the ATPases. ... 

In this chapter, we describe, in a comprehensive way, examples of the recognition, activation, and detection of this important class of biologically relevant anionic substrates. 

Fig. 19. Multiple and multisite substrate recognition (a) a homo dinuclear (dicationic) and (b) a heterodinuclear (cation and anion) iaclusion complex (c) a...

Water molecules or anions close to the active sites in the protease enzymes, mentioned above, may not be considered circumstantial, but may effectively contribute to the removal of the surplus proton from the imidazolium cation before the actual catalytic event. They could serve well to create the initial ion/neutral form of the Aspl02-His57 couple which is important for the initial step of the catalytic process in most discussions 11611 .13i. such a proton removal may be caused by the productive binding of a true substrate (or inhibitor) of the enzyme to the neighboring recognition clefts of the active site. 

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

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